The present invention is generally directed to aircraft icing sensors, and in particular, to a system and methodology for providing among other things as set forth herein, a measure of a water droplet size in a cloud containing liquid water, ice water or both.
Aircraft icing is a serious safety problem for general aviation and some commuter transport airplanes. There has been tremendous growth in the commuter aviation industry in the last few years. When compared to heavy transport category aircraft, these types of aircraft generally operate at lower altitudes and consequently spend a greater proportion of their time operating in icing conditions.
Airframe icing severity depends on the liquid water content, droplet size and degree of glaciation of the cloud. Liquid water content (LWC) is normally expressed in grams/cubic meter and is a measure of the unfrozen water content of the cloud. Droplets occur in nature not in a single size but in a distribution of sizes. The median volume diameter (MVD) of the droplet distribution has historically been used as a bulk measurement of droplet spectrum size. For example, FAA aircraft and engine certification criteria are expressed in terms of a combination of LWC and MVD.
The droplet size plays a critical role in the way that ice accumulates on airframe and engine surfaces. In general the larger the MVD the more problems the accumulated ice will cause. Small droplets tend to freeze immediately on impact whereas larger droplets flow along the structural surfaces before freezing. If the drops have a very large MVD, i.e. greater than 50 microns, they may run past the anti-ice/deice systems and freeze in critical, unprotected places on the airframe. These are the so-called supercooled large drop (SLD) cases. SLD encounters are of particular interest for aviation safety.
Cloud total water content (TWC) includes both unfrozen droplets (liquid water) and frozen water in the form of ice crystals. The degree of glaciation in the cloud can be determined by comparing the liquid water content to the total water content.
In general, airframe and engine icing depend primarily on liquid water content because the unfrozen, but super-cooled droplets, freeze upon impact whereas already frozen ice crystals will not. However, there are some very important exceptions. In some particular cases of temperature and airspeed, ice crystals can melt upon impact and then refreeze. This can cause severe problems for both airframes and engines.
In the past several years there have been a number of fatal commuter aircraft accidents attributed to severe icing conditions having SLD. Though SLD was thought to occur infrequently, the significant increase in commuter aircraft traffic has raised a concern that the chances of encountering this icing condition may be far greater than previously thought. At the present time aircraft ice protection systems are not required to provide protection against SLD. Therefore a simple device that could provide warning that SLD conditions are present would be highly desirable.
Cylindrical and wire like sensors for measuring LWC are known. However, the heating for such known sensors use external windings to heat the sensor. The external windings are quite delicate giving these sensors a very short life due to damage from encounters with typical atmospheric concentrations of ice crystals. Generally speaking sensors using external heater windings are problematic because the windings create small “pockets” that trap ice crystals causing a significant fraction of ice crystals to be captured and evaporated. The evaporation of ice from a liquid water sensor therefore gives false measurements or indications of liquid water when in the presence of ice crystals.
Other prior art LWC sensors have used an internal, switched indirectly heated, cylindrical element to collect super cooled liquid water that freezes onto the cylinder. The cylinder is oscillated at its natural mechanical resonant frequency (approximately 40 kHz). As ice builds up the frequency decreases to a preset value at which time a heating cycle is initiated to clear the ice from the cylinder. The internal heater cycles on and off to remove the accumulated ice. The rate of cycling provided an indication of the amount of liquid water present.
However, it is believed that deficiencies exist in these prior art arrangements, such as the ability to reject unwanted ice crystals when measuring LWC. Tests have shown that the present invention provides a better than ten times improvement over previous LWC sensors in rejecting unwanted ice crystals, due to the shape and surface construction of the sensors.
The development of portions of the present invention has been carried out under a NASA Small Business Innovative Research (SBIR) contract between the NASA Glenn Research Facility and Science Engineering Associates, Inc. (SEA) as part of NASA's Aviation Safety Program initiative to significantly reduce aircraft accidents. Part of this program, is to establish the frequency of occurrence of SLD conditions. Once this frequency of occurrence is established, it would be expected that agencies like the Federal Aviation Administration (FAA) could use this information to affect changes in current aircraft icing certification regulations. This in turn would lead to changes in the design of aircraft ice protection systems, potentially to protect against severe icing conditions.
Present research activities aimed at establishing this frequency of occurrence have been limited to a handful of research organizations using heavily instrumented research aircraft (e.g. the NASA Glenn Research Centers' Twin Otter Icing Research Aircraft and the NRC Convair-580 aircraft). Though effective, this approach is limited in scope and somewhat biased because the research flights are specifically directed into areas having the highest probability of severe icing conditions. Therefore a more extensive approach, such as that offered by instrumenting a large number of commercial/military aircraft, would be of great value. The key to this approach is the development of a small, reliable, low power integrated icing instrument, which has not previously been available.
It is believed that the present invention, in addition to providing needed capabilities in a research environment, will also help provide the needed protection against icing in private and commercial aircraft and overcomes the deficiencies set forth above and achieves the objectives set forth herein.